The invention disclosed herein relates generally to linear motion mechanisms of the type in which successive operations of a plunger alternately cause the plunger to be latched at and released from a latched position, and more particularly, to a compact, low profile electrical switch actuator which is convertible between momentary and alternate action.
Manual keyboards, such as used in office machines, computer terminals and other manually controlled electronic installations commonly require switches having at least two types of switch actions. One type of switch, known as an alternate action switch, operates such that a first depression of the switch actuator or key causes the switch to be actuated to an "on" state, and to remain in that state. A second depression of the switch actuator or key releases the switch and returns it to an "off" state.
The proliferation of apparatus requiring switches for use in keyboards and other switch arrangements has placed increasingly stringent requirements on switches and switch actuators. Keyboard type switches are required to operate reliably over a long life, commonly involving millions of switching cycles. At the same time, the manufacture and sale of switches and switch actuators is highly competitive. Therefore, it is necessary that any alternate action mechanism used in significant quantities be designed for simple manufacture at a minimum additional cost over other types of switches. Among other things, the cost restriction essentially dictates a design which utilizes a minimum number of easily assembled parts.
It is also desirable that an alternate action switch be constructed so that it can be converted to momentary action by a simple modification or adjustment that does not require significant disassembly of the switch or removal of the switch from the panel in which it is mounted. Finally, keyboard designs have largely standardized on key spacing of 0.75 inches center to center, and the trend is toward increasingly low profile keyboard assemblies. Aesthetically, it has been found desirable for keyboard keys to have a somewhat square configuration such that an array of keys substantially completely covers the underlying support panel, and to have sufficient apparent depth to conceal mechanisms other than the key tops mounted thereon. This has been accomplished with a key cap design having a substantially square top with a flaired skirt. In order to meet the low profile requirement any actuator mechanism must fit within the skirt when the key cap is depressed. The fixed spacing and low profile requirements limit the space available for switch actuator mechanisms, including those for achieving alternate action operation. Consequently, such mechanisms must be exceptionally compact.
A variety of mechanisms for achieving alternate action switch operation have been devised. In general, a common feature of these mechanisms is the use of a multi-level cam track or surface on a first major part which is movable relative to a second major part. The second major part carries a cam follower which rides in the cam track. The cam track is typically cardioid or heart shaped in configuration. Operation requires relative movement between the cam track and cam follower both side to side and toward and away from one another. In some designs, the cam follower is biased to a predetermined side to side position. Such a design requires fewer (typically a minimum of two) different levels of the cam track. In other designs in which the cam follower is permitted unbiased side to side motion, more cam track levels (typically at least four) are required.
Various somewhat conflicting design criteria arise in attempting to shrink conventional alternate action mechanisms to fit within the space available on a low profile keyboard, while maintaining long life, reliable operation and low cost. For example, the cross section of the cam follower should be reasonably large to provide adequate stiffness and to minimize stress and wear on the cam surface. However, since the overall width of the cam track may be at least three times the diameter of the cam follower, the follower diameter is limited by the size of the part containing the cam track. Limiting the overall width of the cam track is also desirable from the viewpoint of limiting side-to-side excursions required of the follower and attendant problems in reliably providing for such excursions.
Similarly, deeper cam tracks and steps between adjacent levels generally provide for more reliable operation. However, deeper tracks also require both more uncommitted space in which to form the tracks and more clearance for in-out movement of the cam follower. In addition, deeper tracks necessitate the use of steeper ramps which result in greater operating stresses, and the use of follower biasing springs having longer effective lengths to achieve acceptable biasing forces throughout the longer follower movements.
Although a significant number of different alternate action mechanism designs are known, none of them embody all of the features and characteristics desired for present keyboard switch applications. One representative prior alternate action arrangement is disclosed in U.S. Pat. No. 2,956,446 issued to A. Ensign, Jr., et al on Oct. 18, 1960. The arrangement of this patent employs a cam track formed in a tubular segment rotatably carried on a driver member which slides within a housing. The housing carries a metal guide pin which rides in the cam track. The guide pin is mounted for in-out movement only. A leaf spring which extends parallel with direction of movement of the driver member is provided for inwardly biasing the guide pin. Relative side to side motion is provided by oscillation of the tubular segment on the driver member. Such an arrangement concentrically requires a driver shaft which is big enough to provide adequate strength, surrounded by a tubular segment which is thick enough to accommodate a cam track of adequate depth, surrounded by a housing. Outside the housing there must be adequate clearance for operation of the guide pin. The minimum feasible size of such an arrangement both in cross section and height is too large to meet present low profile keyboard requirements. Finally, the design requires a relatively high parts count which adds to manufacturing complexity and cost.
Another representative prior design is shown in U.S. Pat. No. 3,824,362 issued to G. Bury on July 16, 1974. This patent discloses an alternate action switch mechanism having a molded plastic plunger with a cam slot formed in the side thereof. A housing in which the plunger is mounted has a hole through the wall thereof adjacent the cam slot. A rectangular wire form cam follower is employed. The follower is configured as a rectangle with a portion of one side removed to provide two fingers directed toward one another. One of the fingers is inserted into a dimple in the housing opposite the hole through the wall thereof, and is free to pivot therein. The other finger extends through the hole to engage the cam on the plunger. The cam engaging finger is free to move in and out and oscillate from side to side to follow the cam. In such an arrangement in which a metal cam follower rides on a plastic cam surface, care must be exercised in configuring and finishing the parts to avoid excessive wear and/or gouging of the cam surface.
Yet a further prior design is shown in U.S. Pat. No. 4,001,526 issued to H. Olson on Jan. 4, 1977. This design is relevant because it employs a plastic cam follower molded on the end of a plastic leaf spring. The leaf spring extends parallel with the direction of movement of a plunger of which it is part. Thus, the arrangement is not well adapted to low profile apparatus. Also, as in connection with the Ensign arrangement the leaf spring is designed to permit movement of the cam follower in only one plane. Separate provisions are required for relative movement of the cam track and cam follower toward and away from one another.
The applicants have provided a unique alternate action mechanism of exceptionally simple design constructed of a minimum number of molded plastic parts. The cam surface is formed in the plunger to minimize space requirements while maximizing the amount of material in the plunger shaft to maximize strength. The follower and follower biasing means are molded in the form of a unitary C-shaped collar which is removablely carried in a channel on the housing. The collar configuration permits both side-to-side and in-out follower movement and provides for a very compact mechanization so as to facilitate a low profile keyboard arrangement.